CAES systems store energy by means of compressed air in a cavern during off-peak periods. Electrical energy is produced on peak by admitting compressed air from the cavern to one or several turbines. The power train comprises at least one combustion chamber heating the compressed air to an appropriate temperature. To cover energy demands on peak a CAES unit might be started several times per week. To meet load demands, fast start-up capability of the power train is mandatory in order to meet requirements on the power supply market. However, fast load ramps during start-up impose thermal stresses on the power train by thermal transients. This can have an impact on the power train's life time in that life time consumption increases with increasing thermal transients.
During stand-by, the power train is subject to heat losses and temperature equilibration in the components by heat conduction. An in-flow of cold air through the rotor seals contributes significantly to the heat loss. The longer the stand-by time, the lower component temperatures fall and the larger the thermal stresses become during start-up.
At a commercial CAES power plant in Huntdorf, Germany, the power train consists of two gas turbines with a high and low pressure combustion chamber. The turbines are arranged on a single shaft.
During stand-by, no measures are activated to keep the power train at an elevated temperature. Nevertheless, the power plant is able to start up very fast. This is due to low gas turbine inlet temperatures at full load, which allows an uncooled turbine design and reduces the average thermal transients between start-up and full load and the impact on the rotor's lifetime. However, in view of achieving higher gas turbine efficiencies, this concept of low gas turbine inlet temperatures is no longer suitable.
A further commercial CAES power plant installed in McIntosh, Ala. is similar to the plant in Huntdorf. Its power train comprises a high and intermediate pressure turbine where a combustion chamber is installed upstream of each turbine.
The power train is equipped with a stand-by combustor that is arranged upstream of the high pressure turbine and described in Proceedings of ASME Turbo EXPO 2001, paper 2001-GT-0397. For the purpose of warm-keeping, the stand-by combustor is operated in a continuous or intermittent mode depending on the high pressure casing temperature. Thereby, a minimum temperature of the casings, rotor, stationary and rotating blades, and other components is maintained during stand-by operation and thermal stresses are reduced during start-up.
The stand-by burner is suitable for the prevention of an undesirable power train cool-down. However, the operation of a stand-by combustor for this purpose bears some disadvantages as follows:
Prior to igniting the burner, the system must be purged to fulfill safety requirements. This will consume valuable cavern air.
If purge air cannot be preheated, the purge sequence will remove heat from the turbine. This counteracts the purpose of warm-keeping.
The burner requires a fuel distribution system, which must be considered in the plant's safety concept.
Temperature control is difficult. Direct measurement of flame temperatures is impossible due to high temperatures during burner operation.
The burner emissions can influence the plant's operating permits.
A basic layout of a CAES power plant is shown in FIG. 1. The plant comprises a cavern 1 for storing compressed air. A recuperator 2 preheats air from the cavern 1 before it is admitted to an air turbine 3. The air turbine 3 discharges into the combustion chamber 4, where the air is reheated. The reheated air expands further in the low-pressure turbine 5. Augmented firing in an auxiliary burner 6 can be used to increase the temperature of the exhaust gas before it enters the recuperator 2 on the flue gas side. Following heat transfer to the cold air from the cavern 1, the flue gas leaves the system through the stack 7. The airflow to the recuperator 2 and to the air turbine 3 is controlled by valve arrangements 8 and 9 respectively.